RadioBanter - View Single Post

July 27th 05, 07:35 AM

wrote:

'morning

I took a look, the circuit looks good, so why the better method
comment?

Well, at that time I wanted to experiment how ceramic filters worked, so
I built a very low frequency oscillator and filtered out the wanted
signal near 455k. There is also a slight overhead using just 1/6th of a
TTL chip!

Subsequent studies and experiments taught me that an RC oscillator can
be sufficiently stable at those frequencies. I believe a 455k RC
oscillator can be built right away.

If this 455KHz injected "over the air" signal is captured by the
receiver, should the s-meter provide a constant signal level?

I experimented the external BFO only on those cheap multiband SW radios,
and they have no s-meter.

Moreover I am personally not too interested in meter reading unless I am
measuring something (ie difference between two signals). I prefer to
evaluate a signal by ear.

If you need/want to rely on meter readings, remember that an additional
BFO will give you AM-DSB, while you'll be listening to a SSB signal.
That's half power, the meter should read 3dB less than actual strength
(half point?!).

If the SSB signal is coming in strong (S9), and the captured BFO level
is S3, will the amplitude of the BFO have to be increased?

I do not remember having to deal with BFO amplitude. In "complex"
receivers there might be some implications with AGC, especially if it
follows the 455k mixer (doubt so, it is probably after the 1st conversion).
Assuming the AGC on the 1st IF, then the 455k IF signal level is rather
constant, so BFO amplitude should not need adjustments.

How about AGC on the BFO so that its injection level is a function of
signal level?

That is becoming too complex for a simple solution, IMHO. :-)

Let us know how you decided to proceed and how it worked.

Greetings from the other side of the Atlantic (and a bit of
Mediterranean as well),
Paolo IK1ZYW